Microstrip line

ABSTRACT

The process for the manufacture of the rib waveguide ( 4 ) is based on the process steps (a) ion implantation of high-energy light ions into a crystal, (b) the application of a mask to the surface ( 11.1 ) of this crystal, which defines strips, and (c), the etching of a rib ( 41 ) onto the surface ( 11.1 ) of this crystal. The process step (a) (ion implantation) causes the formation of a layer ( 14 ) with a reduced refraction index at a depth of some micrometers below the surface of the crystal ( 11.1 ). By this refraction index barrier ( 14 ), the light in the direction vertical to the surface of the crystal is restricted. The lateral guidance of the light is effected by the lateral limits ( 43.1 ) and ( 43.2 ) of the rib ( 41 ). 
     The process is in particular suitable for the manufacture of waveguides in non-linear optical crystals, e.g., ferro-electric oxides (KNbO3, LiNbO3, etc.) and borates (LBO, BBO, etc.). These crystal classes have interesting non-linear-optical characteristics and are suitable for utilization as frequency converters. The manufacturing process in accordance with the invention, in particular the combination of ion implantation and the etching of a rib structure, is adapted to the particular characteristics of the material and enables the manufacture of rib waveguides with a low attenuation, whereby the non-linear optical characteristics of the crystal are maintained. This is an important prerequisite for achieving an efficient frequency conversion.

The invention concerns a rib waveguide, a process for manufacturing it,its utilization as well as a light source containing this rib waveguidein accordance with the generic terms of the independent claims.

BACKGROUND

Frequency conversion of laser light by non-linear optical interactionshas been accorded some attention since the sixties. Non-linear opticalprocesses enable the generation of coherent laser light at opticalfrequencies (wavelengths), which cannot, or else only with difficulty,be generated by a direct laser process. In general, for such a frequencyconversion process a laser is utilized as pumping source, the light beamof which propagates through a non-linear optical material. Thenon-linear optical interaction between the laser beam and the materialleads to the effect, that a part of the pumped light is converted intolight of a higher or lower frequency. Among the non-linear opticalprocesses, the second harmonic generation (SHG), sum frequencygeneration (SFG), difference frequency generation (DFG) and opticalparametric amplification (OPA) are of particular significance. Theseprocesses enable the generation of coherent laser radiation in theultraviolet, visible, near—and intermediate infrared spectral range,i.e., between 0.1 μm and 10 μm wavelength. Lasers which emit light atthese wavelengths find applications in spectroscopy, optical datastorage, medicine, biology, etc.

A further important non-linear optical process is the electro-opticalmodulation of laser light. Hereby an electrical field is applied to thenon-linear optical crystal and with this the intensity or propagationvelocity of the laser light is influenced. This effect can be utilizedfor transferring electronic signals to the optical beam. This makespossible the transmission of information with the help of opticalsystems, which today is finding widespread use in communicationstechnology. Apart from this, the electro-optical effect is exploited forvarious other applications, such as in optical switches or in Q-switchesin lasers for the generation of very short laser pulses.

A great number of crystalline materials which are suitable fornon-linear optical interactions have been investigated. Among these,especially the class of the ferro-electric oxides has found attention,e.g., potassium niobate (KNbO₃), lithium niobate (LiNbO3), lithiumtantalate (LiTaO3), barium titanate (BaTiO3) and KTP (KTiOPO4). Ingeneral, crystals of these materials manifest great non-linear opticalsusceptibilities, a material characteristic, which is a necessaryprerequisite for efficient frequency conversion and electro-opticalmodulation. In particular potassium niobate on the basis of itsoutstandingly good characteristics has proved to be an excellentmaterial for non-linear optical applications.

A further class of materials which has interesting non-linear opticalcharacteristics are crystals based on borate compounds, such as β-BaB2O4(BBO), LiB3O5 (LBO), CsB3O5 (CBO) and CsLiB6O10 (CLBO). This group ofnon-linear optical crystals is distinguished by the fact, that itsoptical transparency in contrast to most of the ferro-electric oxidesreaches far into the ultraviolet spectral range. On the basis of thischaracteristic, borate crystals are interesting for frequencyconversions, in which ultraviolet laser radiation is generated.

The degree of conversion in the case of a non-linear process increaseswith the length of interaction of pumped beam and crystal and with theintensity of the pumped beam. In order to achieve a high degree ofconversion, therefore frequently additional measures have to beundertaken, in order to increase the intensity of the pumped beam, thisin particular, if the laser is operated in the continuous wave (cw)mode. Investigated as such measures were above all resonant processes inoptical cavities and conversion processes in optical waveguides.Resonant conversion processes, in the case of which the non-linearoptical crystal is placed in a cavity, provide the possibility ofachieving very high degrees of conversion. However, they have thedisadvantage, that they are very sensitive with respect to themechanical adjustment of the optical components and to smallfluctuations of the pumped beam wavelength. Therefore they normallyrequire a complicated active feedback system for stabilization. Incontrast, the utilization of waveguides for frequency conversion has theadvantage of, solely on the basis of the very small cross sectionalsurface area of the waveguide and of the lateral guiding of the laserbeam, assuring a high intensity over a long interaction length and ofthus achieving a high degree of conversion. For this, already a singlepass of the pumped beam through the waveguide is sufficient, whichsubstantially reduces the demands of the mechanical andfrequency-related stability in comparison with optical cavities.

Waveguides also offer advantages for electro-optical applicationscompared with volume crystals. By the restriction of the light to a verysmall surface over a long distance, the electrical voltage applied forthe modulation can be kept very low, whereby the required electricalpower is significantly reduced. Waveguides are in addition compatiblewith the fibreglass technology, which is utilized in today'scommunications systems.

Of particular significance for applications are rib waveguides, i.e.,channel-shaped waveguides, which guide the light in two directions andlimit it to a very small surface.

The above explanations emphasize the significance of waveguides, inparticular of rib waveguides, for non-linear optical applications. Themanufacturing processes of such waveguides, however, are frequentlytechnologically difficult and have to be adapted to the correspondingmaterial characteristics. This invention is based on a process, whichpermits the manufacture of waveguides of good optical quality innon-linear optical crystals, while maintaining the advantageouscharacteristics of these materials.

For the manufacture of optical waveguides in non-linear opticalcrystals, different methods have been investigated, both chemical—aswell as physical ones. By means of ion diffusion—or ion exchangeprocesses in the non-linear optical crystal, one, for example, succeededin manufacturing waveguides in LiNbO3, LiTaO3 and KTP. Proving to besuccessful, e.g., was the diffusion or implantation of titanium (Ti) inLiNbO3. Within the Ti-doped zone, the refraction index is increased,while simultaneously the desired optical characteristics of the LiNbO3are maintained. Almost all other non-linear optical crystals, however,are not accessible for this process, because the outside ion diffusionconstants and the thermal stability are insufficient. Also the forceddoping with the help of ion implantation does not achieve the objective,because the ion implantation of heavy ions such as Ti damages the hostlattice structure through atom impacts and creates defects, so that nousable waveguides are produced. Among the physical methods, above allthe implantation of light ions such as H+ or He+ have foundapplications. With these processes, the crystalline material issubjected to a bombardment of high-energy ions. This leads to theformation of a buried optical barrier, i.e., to a zone with a loweredrefraction index, and to a wave-guiding layer underneath the surface ofthe crystal.

Fluck and others, in the publication “Low-loss optical channelwaveguides in KNbO3 by multiple energy ion implantation” (J. Appl. Phys.72 (5), 1671 (1992)) have demonstrated, that the manufacture of ribwaveguides by ion implantation, e.g., into ferro-electric oxides such asKNbO3, is possible with a process, which uses several implantationsteps. The rib waveguides manufactured in this manner, however, have thedisadvantage, that they only conduct light of one polarizationdirection, while light with a polarization vertical to that does notpropagate in the waveguide. In order, however, to make a frequencyconversion in KNbO3 possible, the waveguide must necessarily alsoconduct light of both vertical as well as horizontal polarization. Inthe publication “Blue-light second-harmonic generation in ion-implantedchannel waveguides of new design” (Appl. Phys. Lett. 69 (27), 4133(1996)), Fluck and others have described a simpler process, whichenables the manufacture of rib waveguides with only one implantationstep. This process is the subject of the German patent application“Wellen-oder Streifenleiter, sowie Verfahren zu seiner Herstellung”. Ribwaveguides, which are manufactured with this process, have significantlyimproved optical characteristics in comparison with rib waveguides,which are manufactured using several implantation steps. On the basis ofthe narrow optical barrier on the side walls, however, they have acomparatively high attenuation.

In the patent document “Method for the fabrication of low losscrystalline silicon waveguides by dielectric implantation” (U.S. Pat.No. 4,789,642) of Lorenzo and others, a process is described, whichenables the manufacture of rib waveguides by means of ion implantationand etching. In doing so, a silicon substrate is exposed to thebombardment of high-energy oxygen—or nitrogen ions. These ions at acertain depth below the surface of the silicon substrate form acrystalline dielectric layer of silicon oxide (SiO2) or silicon nitride(Si3O4), which in comparison with the silicon layer above it manifests asignificantly decreased refraction index. By etching or epitaxialgrowing of additional silicon in suitable places, subsequently ribs canbe formed on the substrate surface and with this, rib waveguides. Thisprocess, however, is based on the particular material characteristics ofsilicon, where the implantation of heavy ions, such as oxygen ornitrogen effects the formation of a crystalline oxide—or nitride layerand with this a chemical change in the material. In contrast to this,the implantation of light ions, such as protons or helium into aferro-electric oxide, e.g., KNbO3, or a borate, e.g., LBO, leads to theformation of a partially or completely amorphous damage zone due to atomimpacts, not, however, to a chemical change of the material. This isalso applicable for the implantation of heavy ions. The process ofLorenzo and others is therefore not transferrable to ferro-electricoxides or borates. In addition, this patent document refers exclusivelyto waveguides made of silicon for applications in the communicationtechnology at light wavelengths of between 1.3 and 1.6 μm. Because ofthe characteristics of silicon, these waveguides, however, cannot beutilized for frequency conversion processes.

It is the object of the invention to create a rib waveguide of goodoptical quality in a crystal, in particular a ferro-electric oxide or aborate. Furthermore, it is the object of the invention, to indicate aprocess, which enables the manufacture of such waveguides. Furthermore,it is the objective of the invention, to demonstrate the utilization ofthese rib waveguides for the efficient frequency conversion ofsemiconductor diode lasers and of solid-state lasers. In addition, it isthe objective of the invention to create a light source, in which thelight emitted by at least one primary light source is efficientlyfrequency converted.

SUMMARY OF THE INVENTION

The objective is achieved by the rib waveguide, the manufacturingprocess and the light source, as they are defined in the independentclaims.

The rib waveguide in accordance with the invention is manufactured outof a crystal. It manifests a barrier layer in the crystal, therefraction index of which is lower than the refraction index of thecrystal, and which limits the rib waveguide in the direction vertical toa certain surface of the crystal. In addition, it has a rib on thesurface mentioned, the walls of which limit the rib waveguide in thedirection parallel to the mentioned surface.

The process in accordance with the invention to a great extentcircumvents the above mentioned difficulties in the manufacture of ribwaveguides in non-linear optical crystals, such as, e.g., KNbO3. Inessence, it consists of three process steps: (a) the implantation ofhigh-energy light ions into a non-linear optical crystal, (b) thephoto-lithographic production of a mask on the surface of this crystalor the utilization of a suitably pre-structured mask, e.g., made ofsilicon, and (c) the etching of the crystal from the surface, in orderto form a rib screened by the utilization of the mask on this surface,whereby the lateral guidance of the light in the rib waveguide isassured.

The method of ion implantation exploits the permanent radiation damage,which is caused in the base material by the bombardment with ions. Inthe case of the utilization of light ions, e.g., H+, He+ or HE++, theradiation damage caused is concentrated at the end of the ion path. Thezone close to the surface is in comparison only slightly damaged. Thecrystal volume in the depth as a result of the damage manifests a lowerrefraction index. In this way it is possible to guide light between thesurface of the crystal and the damaged volume in the depth. Ionimplantation provides the advantage of being able to manufacturetailor-made waveguides for very specific applications, thanks to theprecisely controllable process parameters (ion dosage and—energy). Thethickness of the wave-guiding layer (position of the optical barrier inthe material) can be defined through the ion energy, while therefraction index in the optical barrier is defined by the ion dosage.

An important prerequisite for the usability of waveguides for non-linearoptical processes consists in the fact, that the non-linear opticalcharacteristics of the material are maintained during the manufacture ofthe waveguides. Precisely ion diffusion—or ion exchange processes canstrongly reduce the non-linearity or destroy it completely. Here too,the ion implantation provides an advantage, because the damage to thematerial in the layer close to the surface, i.e., in the wave guidingzone, is comparatively minor and following the ion bombardment, it canbe partially or even completely reversed.

The utilization of a photo-lithography process for the production of amask for the subsequent manufacture of a rib waveguide, provides thebenefit of being able to structure very precise patterns on the surfaceof the crystal. The photographic lacquer mask can thereupon be hardenedusing special chemical or physical processes, in order to make it moreresistant to the subsequent etching process.

The utilization of a pre-structured mask has the advantage, that themask structure, in particular the mask thickness, can be adapted to thesubsequent etching process independent of the characteristics of thephotographic lacquer. On offer as mask material is in preferencesilicon, also, however, wires or foils made of metal or glass can beused as mask.

The etching of the crystal from the surface and the rib on the surfaceof the crystal created in this manner leads to the manufacture of a ribwaveguide, which manifests important advantages in comparison with thecurrent processes. In particular, with this method the lateral guidanceof the light is not achieved by an implantation process as in the caseof the mentioned processes, which utilize multiple implantations orsingle implantation with inclined side walls. As a result, effects,which have an unfavourable influence on the waveguide, such as, e.g.,tensions in the material, can be avoided. The etching process inaddition can be adapted to the special characteristics of the material,whereby both wet—as well as dry etching processes can be utilized.

The rib waveguide in accordance with the invention can be used inoptical frequency converters, electro-optical modulators and switches.In particular, the rib waveguide in accordance with the invention isutilized in combination with a diode laser or a solid-state laser foroptical frequency multiplication, the generation of sum or differencefrequencies and opto-parametric amplification or oscillation.

The rib waveguide in accordance with the invention is suitable forapplications in optical frequency converters, electro-optical modulatorsas well as switches. For example, through the combination of a ribwaveguide manufactured in KNbO3 and an AlGaAs or InGaAs diode laser, byoptical frequency doubling light in the spectral range between 430 and550 nm can be generated. As further possible pumped light sources, apartfrom semiconductor diode lasers also solid-state lasers are possible,above all Nd and Cr doped garnet, such as, e.g., YAG (Y3A15O12), GGG(Gd3Ga5O12), YSAG (Y3Sc2Al3O12), Gsag (Gd3Sc2Al3O12), GSGG(Gd3Sc2Ga3O12) as well as mYVO4, LiSAF and Ti:Al2O3. As a furtherexample, through the combination of a diode laser, e.g., AlGaInP and awaveguide in a borate crystal, ultraviolet laser radiation in thewavelength range between 180 and 430 nm can be generated by frequencydoubling. As further possible pumped light sources, apart from diodelasers also frequency-doubled solid-state lasers are possible. Boratecrystals are suitable for non-linear optical processes of a higherorder, e.g., the generation of third—or fourth harmonics, which, e.g.,can be pumped with the mentioned solid-state lasers. Apart from this,with two light beams, which originate from two different laser sources,in a waveguide the generation of sum—or difference frequency can beachieved. As a further application, the waveguide can be utilized forthe opto-parametric oscillation. These processes enable the generationof ultraviolet, visible or infrared light tuned to the wavelength, bypumping with a suitable laser. The rib waveguide is thereupon suitablefor applications in communication technology, such as in electro-opticalmodulators, switches or directional couplers, whereby the waveguide iscombined with suitable electrode structures. These components, forexample, enable the optical coding of information onto a light beam orthe switching of light between different glass fiber waveguides.

The light source in accordance with the invention contains at least oneprimary light source and at least one frequency converter, into whichlight emitted from the primary light source is coupled, whereby at leastone frequency converter is the rib waveguide in accordance with theinvention. Preferably suitable as primary light sources aresemiconductor diode lasers (e.g., AlGaAs or InGaAs diode lasers),solid-state lasers (e.g., Nd:YAG, Nd:YO4 or Cr:LiSAF), wave guide lasersor fibre lasers. The light sources, which are based on the opticalfrequency multiplication (e.g., frequency doubling), opto-parametricamplification or opto-parametric oscillation, in preference contain aprimary light source and a frequency converter. Light sources, which arebased on optical sum—or difference frequency generation, in preferencecontain two primary light sources and a frequency converter.

The light emitted by the primary light source is preferably input to therib waveguide directly or through an optical system. In order to keepthe coupling losses at the first and second front side of the ribwaveguide as low as possible, in preference both front sides of the ribwaveguide should be provided with an anti-reflex coating. The residuallight from the primary light source is uncoupled together with the lightgenerated in the rib waveguide by the frequency conversion, e.g., lightwith the double frequency, through the second front side of the ribwaveguide and in preference by means of an optical system, e.g., one orseveral lenses, bundled into a laser beam with a small divergence. Thesecond front side of the rib waveguide can also be provided with areflecting coating, so that the light coupled into the rib waveguidetogether with the light generated by the frequency conversion passesthrough the rib waveguide once again in the other direction and is thenuncoupled through the first front side of the rib waveguide.

In the preferred embodiment, the crystal, (e.g., KNbO3) is bombardedwith high-energy light ions (e.g., He+ ions with 2 MeV energy), in orderto form a damaged zone buried underneath the surface with a lowerrefraction index. Subsequently, a mask made out of photographic lacqueris produced with the help of a photo-lithographic process, which coversnarrow, strip-shaped zones. Finally the crystal is etched from above,whereby the zone covered by the mask remains unchanged. As a result, arib is formed in this zone, which stands out from the surface of thecrystal. Utilized for the etching is in particular a physical etchingprocess (sputtering), for example, plasma etching with Ar+ ions.

In a second embodiment, following the process step (a) (implantation ofa planar waveguide) the crystal is baked for a certain time period, inorder to cure defects in the crystal lattice structure, which occurduring the implantation. This curing process can also be carried outafter the process step (c) (etching of a rib).

In a third embodiment, providing the non-linear optical crystal isferro-electric, following the process step (a) (implantation of a planarwaveguide) an external electric field is applied along the spontaneouspolarization of the crystal, in order to reorientate ferro-electricdomains created during the implantation and to pole the crystalcompletely along a designated direction.

In a fourth embodiment, following the process step (b)(photo-lithographic production of a mask) a process is utilized, whichhardens the photo-lithographic mask and therefore makes it moreresistant against the subsequent etching process. This process canconsist of an additional implantation step with ions of low energy. Asfurther possible measures for hardening the mask, chemical processes,baking or irradiation with ultraviolet light can be applied.

In a fifth embodiment, following the process step (c) (etching of a rib)a layer of low refraction material is applied to the surface of thecrystal. This layer can in particular consist of oxidic compounds, suchas SiO2, Al2O3, Ta2O5 or Nb2O5. This layer on the one hand serves toprotect the rib waveguide from mechanical damage, on the other hand ithas the purpose of reducing the optical diffusion losses at theinterface waveguide-air.

In a sixth embodiment, the rib waveguide is split-up into two or morezones in such a manner, that light can be guided separately in each oneof these zones. This is achieved by the implantation or further indexbarriers, whereby this takes place following the process step (a)(implantation of a planar waveguide) or (c) (etching of a rib).

In a seventh embodiment, following the process step (a) (implantation ofa planar waveguide) a pre-structured mask is utilized instead of themask made of photographic lacquer. This pre-structured mask inpreference consists of silicon, however, wires, fibres or foils made ofmetal or glass can also be utilized.

DESCRIPTION OF THE DRAWINGS

The invention and as a comparison the state of the art are explained indetail on the basis of the FIGS. 1 to 14. These show:

FIG. 1 the manufacture of a planar waveguide in a crystal by means ofion implantation in accordance with the state of the art,

FIG. 2 a mask for the subsequent etching of a rib waveguide,

FIG. 3 the etching of a rib on a crystal,

FIG. 4 a rib waveguide created by ion implantation and the etching of arib on a crystal, whereby the index barrier lies below the rib,

FIG. 5 a rib waveguide, in the case of which the index barrier is withinthe rib,

FIG. 6 the process for re-poling a ferro-electric crystal, in order tomake it single domain,

FIG. 7 the process for hardening the photographic lacquer mask for thesubsequent etching process by means of ion implantation,

FIG. 8 a rib waveguide with a covering layer applied on top,

FIG. 9 a crystal with several rib waveguides,

FIG. 10 a rib waveguide, which is split-up into several zones, whichseparately guide light,

FIG. 11 a light source, which utilizes a semi-conductor diode lasertogether with the rib waveguide in accordance with the invention as anefficient frequency converter,

FIG. 12 a compact light source, which utilizes a semi-conductor diodelaser together with the rib waveguide in accordance with the inventionas an efficient frequency converter,

FIG. 13 a light source, which utilizes a solid-state laser together withthe rib waveguide in accordance with the invention for an efficientfrequency conversion,

FIG. 14 a light source, which utilizes the rib waveguide in accordancewith the invention together with a waveguide laser for efficientfrequency conversion.

For reasons of clarity, the proportions in the Figures do not correspondto those in reality.

DETAILED DESCRIPTION

The state of the art for the manufacture of a planar waveguide by meansof ion implantation is illustrated in FIG. 1. A non-linear opticalcrystal 11 (e.g., KNbO3) is exposed to an ion beam 12 of high-energylight ions. Considered as such ions are, e.g., protons (H+) or heliumions (He+ or He++) with energies in the range of some tenths of—to someMega-electron Volts (MeV), e.g., 0.3 to 3 MeV. The ions penetrate intothe crystal and due to the interaction with the electrons and atoms ofthe crystal lose their energy. In a zone close to the surface 13, thecrystal is damaged comparatively slightly. At the end of the ion path, azone 14 is formed, within which the crystallinity of the material isdamaged or destroyed. Because of this damage, the zone 14 has a lowerrefraction index than the undamaged crystal 11. With this, in the damagezone a refraction index barrier 14 versus the zone close to the surface13 above it and the undamaged crystal volume below it is formed. Therefraction index barrier 14 makes it possible to guide light in the zoneclose to the surface 13 above it. The typical thickness d1 of this zone13 amounts to some micrometers (μm), e.g., 4.4 μm for 2 MeV He+ ions inKNbO3, while the thickness d2 of the damaged zone 14 (refraction indexbarrier) amounts to some tenths of μm. The complete structure,consisting of zone 13 close to the surface, refraction index barrier 14and the undamaged crystal volume 15 serving as substrate, form a planarwaveguide 1. In order to increase the thickness d2 of the barrier layer14, it is also possible to carry out several implantation steps withions of slightly differing energy. For example, in KNbO3, two subsequentimplantation steps with ions with an energy of 2 and of 1.85 MeV lead tothe formation of a 0.8 μm wide damaged zone. By the manufacture of awide refraction index barrier 14, the optical losses of the waveguideresulting from the radiation of light out of the waveguide into thesubstrate can be prevented. The crystal 11, which is utilized for themanufacture of the waveguide, in preference has surfaces polished tooptical quality. Apart from this, however, the process can also beutilized on crystals with naturally grown surfaces. The edges of thecrystal relative to the crystallographic axes can have any direction.

The process in accordance with the invention is described in the FIGS. 2to 10. After the manufacture of a planar waveguide 1 in a non-linearoptical crystal 11 by means of ion implantation (FIG. 1), a strip-shapedmask 21 made of photographic lacquer or of another suitable material isapplied to the surface of this planar waveguide, as is illustrated inFIG. 2. The width b of the strip amounts to some micrometers (e.g., 5μm). The steepness of the flanks 22.1 and 22.2 of the strip-shaped mask21 can be changed through the exposure—and development time of thephotographic lacquer during the photographic process, whereby inpreference an a great as possible steepness is strived for, i.e., a ribwith vertical flanks. It is also possible to utilize a prestructuredmask. This provides the benefit, that the mask structure, in particularthe mask thickness and the flank shape, can be adapted independent ofthe processing characteristics of the photographic lacquer. On offer asmask material is in preference silicon, but also wires or foils made ofmetal or glass can be utilized as mask. FIG. 3 illustrates the nextprocess step. By means of a dry etching process, the zone 1 close to thesurface is etched down with ions 31 in the two not covered zones 32.1and 32.2. For KNbO3, for example, for this in preference Ar+ ions from aplasma are utilized, whereby the material is mechanically sputtered off.The energy of these ions preferably lies in the range of 0.1 to 20 kiloelectron Volts (keV). As a further possibility, ions of chlorinefluoride hydrocarbon compounds (e.g., CH3F, CC12F2, CF4) can be made useof as chemical dry etching agents. The etching process can also becarried out as a wet etching process, e.g., by etching with hydrofluoricacid (HF). By the etching, the uncovered zones 32.1 and 32.2 are loweredrelative to a covered zone 33, and a rib is formed on a defined area11.1 of the crystal 11. A rib 41 like this is depicted in FIG. 4. Thewhole structure consisting of the rib 41, two zones 42.1 and 42.2laterally adjacent to the rib, the refraction index barrier 14 as wellas the crystal volume 15 serving as substrate form the rib waveguide 4.In this, d3 designates the distance between the crystal surface 11.1 andthe index barrier in the lateral zones 42.1 and 42.2 and d4 the distancebetween the surface of the crystal and the index barrier 14 in the rib41. The rib waveguide 4 guides light in two directions, whereby theguidance vertical to the crystal surface 11.1 is enabled by therefraction index barrier 14 and the guidance in the direction parallelto the crystal surface 11.1 by the lateral limits 43.1 and 43.2 of therib 41. The height h of the rib 41, defined as the distance between thesurface of the rib 41 and the surface 11.1 of the crystal above thelateral zones of the rib 42.1 and 42.2, is determined by the duration ofthe etching. In order to assure the lateral guiding in the rib waveguide4, already a small height h (etching depth) in comparison to thedistance d4 of the rib surface to the index barrier 14 is sufficient.For example, for a thickness d4 of 5 μm, a rib height of approximately0.5 μm is sufficient for achieving the lateral guidance of the light inthe rib waveguide 4. To be strived for is an etching depth h of between25% and 75% of the thickness d4, in order to achieve an optimum lateralguidance. The duration of the etching, however, can also be selected insuch a manner, that the height h of the rib 41 is equal to or greaterthan the distance d4 of the rib surface from the refraction indexbarrier 14. FIG. 5 shows a rib waveguide 4, where h is greater than d4.

The bombardment of a crystal 11 with light ions 12 (FIG. 1) can lead todefects in the crystal lattice structure in the irradiated zone 13 closeto the surface. These defects concern, e.g., the displacement ofindividual ions from their positions in the lattice structure or theformation of oxygen vacancies in the lattice structure. These defectscan absorb or scatter light and therefore cause undesired optical lossesof the light guided in the rib waveguide. In many non-linear opticalcrystals, such defects can be cured by a heat treatment (tempering) ofthe crystal following the ion implantation. In KNbO3, for example, sucha tempering treatment at 180° C. for a period of ten hours leads to areduction of the optical losses of the waveguide by up to 10 dBcm−1.Such a tempering step can also only be carried out after the formationof the rib waveguide 4 (FIG. 4). The tempering step can be carried outin a normal atmosphere. For the curing of oxygen vacancies, it can,however, also be advantageous to carry out the tempering treatment in anatmosphere with an excess of oxygen.

In the particular case of ferro-electric crystals, the centers ofgravity of the positively and negatively charged ions do not coincide,as a result of which the material obtains a spontaneous polarization Ps,which characterizes a preferred direction in the crystal. In an idealferro-electric crystal, the spontaneous polarization has the samedirection in the whole crystal. Such a crystal is designated as singledomain. If in a ferro-electric crystal several zones occur, in which thespontaneous polarization has different directions, then the crystal isdesignated as multi-domain or depolarized. The bombardment of aferro-electric crystal 11 with light ions 12 leads to the formation offerro-electric domains in the irradiated zone 13, i.e., to zones, whichhave a differing direction of polarization. Such a depolarization canlead to the partial loss of the optical non-linearity in the zone closeto the surface 13. FIG. 6 illustrates a possible arrangement, by meansof which through the application of a sufficiently high externalelectric field to the crystal (e.g., 3 kVcm−1 in the case of KNbO3), thepolarization in these domains can be fully aligned in parallel again.Such a post-poling process is in preference carried out after theimplantation of the planar waveguide 1. Indicated is the preferreddirection of the spontaneous polarization Ps of the original crystal 11used for the manufacture of the waveguide. With the help of a highvoltage source 61, which is only schematically depicted, and of anelectrode 62.1 and 62.2 each respectively applied to the top and bottomside of the crystal 11, a DC voltage V is applied to the crystal 11 andan electric field with its direction parallel to Ps is created. Underthe influence of this field, the spontaneous polarization in domains inthe zone close to the surface 13, which have been created as a result ofthe implantation 12 and the polarization vector of which is not anymoreparallel to Ps, is re-aligned along the direction of Ps. Since thedirection of Ps does not necessarily have to run vertical to the crystalsurface, also other arrangements than the one illustrated in FIG. 6 arepossible. For example, Ps can also run parallel to the crystal surface,so that the electrodes 62.1 and 62.2 correspondingly are applied to thelateral surfaces of the crystal 11. The electrodes 62.1 and 62.2 canconsist of metallic plates, which are brought into contact with thecrystal 11. Electrodes can equally be produced by the application of ametallic layer (e.g., by vaporization or painting on). Furthermore,electrolytic liquid electrodes can be utilized. Possible is also acombination of different types of electrodes, for example, a metallicplate underneath and a vaporized metal layer on top. The voltage V canbe applied both in static form as well as pulsed.

A further manufacturing step can be inserted after the formation of thestrip-shaped photographic lacquer mask 21 (FIG. 2). By means of afurther bombardment with light ions of low energy 71 (between 0.1 and0.5 MeV for helium, between 0.05 and 0,2 MeV for protons), it can beachieved, that the ions are already completely moderated in the mask 21,as is shown in FIG. 7. This leads to a structural change of thephotographic lacquer, which results in a hardening against mechanical orchemical processing. In two not covered zones 72.1 and 72.2, the ionsbecause of their low energy only penetrate into a surface 73 immediatelybelow the surface of the crystal 11. A hardening of the photographiclacquer has the consequence, that the etching rate ratio of thephotographic lacquer relative to the crystal can be improved. This is ofgreat importance, because the etching rates of the crystal 11 can bevery small and one therefore has to prevent the photographic lacquermask 21 being eroded too rapidly relative to the crystal during theetching process. To be considered are also other hardening measures,such as baking the photographic lacquer at an increased temperature,irradiation with UV light or chemical processes, such as, e.g. thetreatment of the photographic lacquer with chlorobenzene.

FIG. 8 demonstrates how after the etching process a covering layer 81 isapplied onto the rib waveguide and the laterally adjoining zones 43.1and 43.2. This consists of a dielectric material, which has a lowerrefraction index than the zones 41, 43.2 and 43.2. For this, inparticular oxides such as SiO2, Al2O3, Ta2O5 or NbO5 come intoconsideration. The covering layer 81 can be applied with a conventionalprocess, such as, for example, vaporization or sputtering under vacuum.Its task on the one hand consists in the protection of the rib 41 frommechanical damage. On the other hand, by the reduction of the refractionindex jump at the interface surfaces between rib waveguide 4 and theambient, the optical diffusion losses of the waveguide are reduced.

FIG. 9 illustrates, how in a crystal 11 several, e.g., two, ribwaveguides 4.1 and 4.2 can be produced by the etching of several ribs.Such arrangements can, for example, find application in directionalcouplers, optical switches or integrated optical interferometers. In it,the distance a of the rib waveguides amounts to at least half the ribwidth b and at most the whole width of the crystal B. The process steps(a) (implantation), (b) (photo-lithographic mask production) and (c)(etching) in preference take place simultaneously for all waveguides,whereby in the case of the process step (b), a mask with a suitablestrip pattern is utilized. The number of waveguides on a crystal canalso be significantly greater than in the example illustrated, e.g.,also 100.

For applications in optical switches it can be necessary to producestructures with several waveguides lying one above the other, in orderto switch light from one channel to another. FIG. 10 shows such anarrangement of two superimposed, separately guiding zones 101.1 and101.2 within one rib 41. The two zones 101.1 and 101.2 are separated byan index barrier 102, which was also produced through ion implantation.This additional implantation step can take place after the manufactureof the originally planar waveguide 1 (FIG. 1). In doing so, for thesecond implantation step ions with low energy are utilized, e.g., withhalf the energy, which was utilized for the prosduction of the planarwaveguide 1. Subsequently, the etching process as described above iscarried out. Equally, however, the second implantation can also onlytake place following the etching of a rib (FIG. 4). Furthermore, it ispossible to produce several, e.g., eight, separately light-guiding zones101 with more than two successive implantation steps.

The rib waveguide in accordance with the invention is in preferenceutilized in combination with a semiconductor diode laser or asolid-state laser for optical frequency multiplication, sum-— ordifference frequency generation and opto-parametric amplification oroscillation. Preferred embodiments of light sources, which contain thewaveguide in accordance with the invention, are described in the FIGS.11 to 14.

FIG. 11 and 12 show, how the combination of a diode laser 111 with a ribwaveguide 4 results in a compact frequency-converted laser. To achievethis, the rib waveguide in accordance with the invention 4 is combinedwith a semiconductor diode laser in such a manner, that the light fromthe diode laser by means of an optical system, for example, one orseveral lenses 112 (FIG. 11) or also directly is coupled into thewaveguide 4 and frequency-converted. It is also possible to utilizeseveral primary light sources (in preference two). The first and/orsecond front side 113, 114 of the rib waveguide 4 can be provided withreflecting—or anti-reflex coatings. A reflecting coating on the secondfront side 114 makes it possible, that the rib waveguide 4 in a secondpass can be utilized again for frequency conversion (also possible forthe embodiments of FIGS. 13-14).

FIG. 13 illustrates how the combination of a solid-state laser 121 witha rib waveguide 4 results in a compact frequency-converted laser. Toachieve this, the rib waveguide 4 is combined with a solid-state laserin such a manner, that the light from the solid-state laser 121 by meansof an optical system, e.g., one or more suitable lenses 122, is coupledinto the waveguide 4 and frequency-converted.

FIG. 14 illustrates how the combination of a waveguide laser 132 with arib waveguide 4 results in a compact frequency-converter laser. Toachieve this, the rib waveguide is combined with the waveguide laser 132in such a manner, that the light from the waveguide 132 is directlycoupled into the waveguide 4 and frequency-converted. Suitable aswaveguide laser is also a fibre laser.

What is claimed is:
 1. An apparatus comprising a waveguide, thewaveguide manufactured from a non-linear optical crystal having a firstsurface, with a barrier layer in the crystal, the refractive index ofwhich is lower than the refractive index of the crystal and which limitsthe waveguide in a direction vertical to the first surface, the materialof the crystal being chemically unchanged in the barrier layer, butphysically changed, inasmuch as the barrier layer contains ions andmanifests deliberate radiation damage caused by the presence of theions, as a result of which its refractive index is lower than that ofthe crystal, the waveguide further designed as a rib waveguide with arib on the first surface, the walls of the rib limiting rib waveguide ina direction parallel to the first surface and the height of the ribabove the first surface amounts to at least 0.5 micrometer.
 2. Theapparatus in accordance with claim 1, wherein the crystal is aferro-electric oxide or a borate.
 3. The apparatus in accordance withclaim 1, wherein at least one dielectric covering layer, the refractiveindex of which is lower than the refractive index of the crystal, on thesurface of the crystal.
 4. The apparatus in accordance with claim 1,wherein the rib in the direction vertical to the crystal surface issplit-up into two or more zones, which are separated by one or morebarrier layers.
 5. An apparatus comprising an optical frequencyconverter for frequency multiplication, sum frequency generation,difference frequency generation or opto-parametric oscillation or inelectro-optical modulators, switches or directional couplers, whereinthe optical frequency converter possesses a rib waveguide in accordancewith claim
 1. 6. An apparatus comprising a light source with at leastone primary light source and at least one frequency converter, intowhich light emitted from the at least one primary light source can becoupled, wherein the at least one frequency converter is a rib waveguidein accordance with claim
 1. 7. The light source in accordance with claim6, wherein the at least one primary light source is a semiconductordiode laser, a solid-state laser, a waveguide laser or a fibre laser. 8.The light source in accordance with claim 6, wherein the light emittedby the at least one primary light source can be coupled into the atleast one rib waveguide directly or through an optical system.
 9. Thelight source in accordance with claim 6, wherein the at least one ribwaveguide has a first and second front side and that at least one ofthese front sides is provided with a reflecting or anti-reflex coating.10. A process for manufacturing a waveguide from a non-linear opticalcrystal, comprising: (a) producing, as a first step, a barrier layer inthe crystal, which in essence is parallel to a certain surface of thecrystal, in that ions with a defined energy distribution and a defineddosage are implanted into the crystal through the certain surface, thatthe ions to be implanted, the distribution of the implantation and theimplantation dosage are selected in such a manner, that the implantedions do not form a chemical compound with the material of the crystal,do, however, physically change the crystal within a damage zone,inasmuch as the ions implant themselves in the damage zone and theredeliberately cause permanent radiation damage, as a result of which therefractive index of the damage zone is permanently lowered relative tothe crystal; (b) applying a mask on the surface mentioned of thecrystal; and (c) etching of the crystal from the surface mentioned, inorder to form a rib on the surface mentioned, to such an extent, untilthe height of the rib above the surface mentioned amounts to at least0.5 micrometers.
 11. The process in accordance with claim 10, whereinfor process step (a), the barrier layer is produced by the implantationof high-energy light ions into the crystal.
 12. The process inaccordance with claim 11, wherein for process step (a), He+ ions, He++ions or H+ ions with an energy of 0.3 MeV to 3 MeV are utilized.
 13. Theprocess in accordance with claim 10, wherein following the process step(a) or (c), the crystal is baked at an increased temperature, in orderto cure defects in a zone between the surface mentioned and the barrierlayer.
 14. The process in accordance with claim 10, wherein followingthe process step (a) or (c), an external electric field is applied tothe crystal, in order to re-orientate ferro-electric domains and to polethe crystal along a designated direction.
 15. The process in accordancewith claim 10, wherein for process step (b), the mask on the surfacementioned is produced by a photo-lithographic process.
 16. The processin accordance with claim 15, wherein for process step (b), the mask ishardened by means of ion implantation, chemically, by baking or byirradiation with ultraviolet radiation.
 17. The process in accordancewith claim 10, wherein for process step (b), the mask is separatelypre-structured and placed on the surface mentioned.
 18. The process inaccordance with claim 10, wherein in a crystal several rib waveguidesare simultaneously produced.